The present invention relates in general to a method for acoustic well logging and, more particularly, to a method for detecting the horizontal azimuthal direction of the maximum and minimum earth stresses in subsurface formations behind well casing.
It has long been known to acoustically log open wellbores to determine the velocities of compression ("P") waves and shear ("S") waves traveling through rock formations located in the wellbore region and tube waves ("T") traveling along the wellbore interface. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at pre-selected distances from the sound source.
By timing the travel of compression waves, shear waves, and/or tube waves between the source and each receiver, it is normally possible to determine the nature of surrounding rock formations including natural fracture identification. For descriptions of various logging techniques for collecting and analyzing compression wave, shear wave, tube wave, and secondary wave data, please refer to U.S. Pat. Nos. 3,333,238 (Caldwell); U.S. Pat. No. 3,362,011 (Zemanek, Jr.); Reissue No. 24,446 (Summers); 4,383,308 (Caldwell); 4,715,019 (Medlin et al); and U.S. patent application Ser. No. 192,446 (Medlin); and to "The Correlation of Tube Wave Events With Open Fractures in Fluid-Filled Boreholes" by Huang and Hunter in Geological Survey of Canada, pgs. 336-376, 1981.
In each of the foregoing references, the acoustic waves are generated in the formation in response to an acoustic energy transmission from within an open wellbore. However, the teachings of such references are not applicable to the identification of earth stress orientation in formations which are traversed by well casing, that is, well pipe cement bonded to the formation. An acoustic source within the cased well generates tube waves which travel along the well casing. These cased well tube waves are the predominant mode of wellbore excitation at low frequencies of below about 2 kHz. Their amplitudes are orders of magnitude greater than those of compressional or shear waves generated in the casing or formation. In cemented intervals the cased well tube waves can be sensitive to changing conditions behind the casing.
It is therefore a specific objective of the present invention to provide for a method of logging a cased well to detect earth stress orientation in the formations surrounding the cased well.